The present invention relates to a method of producing resin varnish for printing ink, and particularly relates to a method of productively producing a resin varnish with superior high-speed printability, a high molecular weight, a high softening point, and a high viscosity.
A rosin-modified phenolic resin is used widely as the principal component of the ink in offset printing, a representative printing method.
In the general conventional method of producing a rosin-modified phenolic resin, a phenolic resin and polyvalent alcohol are added to a molten resin, and phenol is added at an elevated temperature from 230° C. to 270° C., after which an esterification reaction is performed.
In this bulk polymerization method, a high temperature is required to accelerate the reaction between the rosin and polyvalent alcohol and to produce a high-molecular-weight resin in the molten state.
In recent years, to improve printed material productivity, the printer speed has been increased and inks for high-speed printing have been developed, so even in a rosin-modified phenolic resin, which is a resin for ink, high-speed printability (e.g., misting control, quick drying, in-printer stability, emulsification resistance) is demanded.
To achieve such performance, it is necessary to produce a resin with a high molecular weight, a high softening point, and a high viscosity. However, when an attempt is made to use a conventional bulk polymerization method to produce a rosin-modified phenolic resin with a high molecular weight, the increase in the melting viscosity causes nonuniformity within the system and an excessive stirring load, or there are problems when removing the molten resin from the reaction system, thereby limiting the molecular weight, softening point, and viscosity of the producible resin.
Also, a gel varnish for printing is prepared by dissolving the obtained rosin-modified phenolic resin in an ink solvent and drying oil by means of the following process, and by then adding a gelling agent (gellant), as required, and then cooking. However, a problem such as the following results: When a gel varnish is prepared by redissolving a solid varnish resin, the high-molecular-weight resin decomposes readily and the low-molecular components resulting from decomposition adversely affect the ink performance and the reaction with the gelling agent is considerable, so it becomes difficult to control the production of gel varnish. In Japanese Laid-Open Patent Publication Number 10-88052 (hereinafter, Prior Art 1), the present applicant previously disclosed a method of producing a resin varnish by subjecting phenol and formaldehyde to a condensation reaction in xylene to produce a phenol-formaldehyde initial condensate (hereinafter, resole initial condensate), and by then reacting in an ink solvent this resole initial condensate and a rosin ester. (See claim 1 and paragraphs 30 and 31 of Prior Art 1.)
Also, in Japanese Laid-Open Patent Publication Number 2001-261768 (hereinafter, Prior Art 2), a method of producing a resin varnish in the presence of a mixed basic oxide catalyst of lithium oxide and barium oxide, by adding a rosin, formaldehyde, phenol, and a polyol to drying oil or semidrying oil, or by further adding this to a solution mixed with a petroleum solvent is disclosed. (See the claims and paragraph 23 of Prior Art 2.) In the production method of the aforementioned Prior Art 1 and 2, in contrast with the conventional bulk polymerization method in which the reaction system readily becomes nonuniform, because of the solution polymerization reacted in the solvent system, the viscosity of the entire system is reduced and it becomes uniform, so it becomes easy to stably and economically produce a high-viscosity, high-molecular-weight resin by reducing the reaction temperature.
However, as is evident from the aforementioned Prior Art 1, for example, when a rosin ester (gum rosin ester is used all embodiments) and the resole initial condensate are reacted in a solvent, because of various factors attributable to rosin characteristics such as the type of rosin and the degree of rosin modification, differences occur in the reactivity of the added phenol with the rosin ester, and this difference causes significant fluctuation of the resin's viscosity and molecular weight, so there is considerable scope for improving the production of an ink resin with superior printability. The technical challenge of the present invention is to highly efficiently produce a resin with superior high-speed printability, high viscosity, and high molecular weight, by further improving the aforementioned Prior Art 1 to increase the reactivity between the rosin ester and the resole condensate.